Ultrasound occupies an important role in cardiac, fetal, and breast imaging, among other applications. For example, ultrasound's real-time nature and lack of ionizing radiation can make it more attractive than other alternatives. Unfortunately, high levels of image clutter can present a significant problem for certain patients, and diffraction effects can limit spatial resolution (e.g., to no better than hundreds of microns). For example, three-dimensional dynamic focusing can be needed to approach the predicted theoretical diffraction limit. Such focusing can unacceptably increase cost and complexity of the imaging system.
Generally, the resolution limit for ultrasound is assumed to be no better than λz/D, where λ can represent the ultrasound acoustic wavelength, z can represent the range to a target to be imaged, and D can represent an aperture size corresponding to the ultrasonic transducer. Thus, at least two routes can be used to improve resolution. The first can be to increase the operating frequency and thereby reduce the wavelength, λ. Such wavelength reduction works well at shallow depths, but can be limited by frequency dependent attenuation as the depth of the region of interest is increased. As the operating frequency increases, the signal to noise ratio (SNR) can also decrease, until it falls too low to form useful images. In one approach, coded excitation can mitigate this effect, but a tradeoff between resolution and SNR still exists. In another approach, resolution can be increased by expanding the imaging aperture, at least up to the point where an f-number (e.g., z/D) approaches 0.5.
While aperture growth can be broadly effective, a practical limit can exist since the aperture cannot be made unreasonably large. Research over the past decade has shown that resolution in vivo does not necessarily scale with aperture growth as predicted. This effect is believed to result from phase aberration. For example, as larger apertures are employed, the effect of phase aberration can become more significant. Moreover, for many organs the anatomical acoustic window may not be large enough to allow f/0.5 imaging. For example, in heart imaging, the space between the ribs can limit useable aperture size.
Aperture growth in elevation can also have only limited value in linear and phased arrays that use a fixed lens for elevation focusing. For example, a larger aperture in elevation can improve resolution at the focus, but it can also degrade resolution elsewhere. In another approach, if the active aperture size is increased while maintaining element spatial sampling, then a channel count used for the imaging system may grow exorbitantly. This is especially true when 1.5D or 2D arrays are employed for elevation focusing.
The past decade has seen 2D arrays progress from research curiosities to common clinical tools. The majority of 2D array applications have been in cardiology where there is a need for rapid volume acquisition. Such 2D array systems can include 2,500 active elements or more, and can include integrated circuitry within the probe assembly. Such 2D arrays can use sub-array beamforming techniques, but such methods generally limit resolution and contrast to below theoretical limits.
Extending these approaches to breast imaging can also present major challenges. For example, the operating frequency in the breast can be substantially higher. Given the spatial sampling requirements of conventional beamforming (CBF) this might entail a quadrupling (doubling in each dimension) of channel count for a similarly-sized transducer as would be used for a cardiac imaging application. Also, to approach diffraction limited resolution, a 2D array for breast imaging can be twice as large in each dimension as compared to cardiac imaging. Thus to a crude degree, a 2D array based breast imager can be roughly 16 times the elements and channels of existing 2D arrays. Generally, cardiac 2D arrays have costs that can exceed tens of thousands of dollars, so a 2D array based breast scanner might not be commercially viable, unless channel count or array size can be reduced.